I don't know where you're writing from, but in almost every part of the world, the law does not protect speech that is insulting or expresses ridicule. This kind of speech is typically classified as defamation https://en.wikipedia.org/wiki/Defamation: When written, it's known as libel, and when spoken, slander.
There are some differences in how different nations understand defamation, but in general terms, it's understood as the expression of a false statement, known or believed by the speaker to be false, aimed at harming the reputation of a person or group. This definition suggests that the philosophical rationale for defamation not being legally protected is along the lines you suggest, namely, that it's not "constructive." Generally speaking, we do not much benefit by believing what is false, so the audience for defamatory speech does not benefit from it. Moreover, defamatory speech does not contribute to public discourse and is not even intended to advance our knowledge of the truth. Hence,...